The internal structures of houses and other buildings are commonly protected from environmental elements by exterior siding materials. These siding materials are typically planks or panels composed of wood, concrete, brick, aluminum, stucco, wood composites, or fiber-cement composites. Some common fiber-cement composites are fiber-cement siding, roofing, and trim which are generally composed of cement, silica sand, unbleached wood pulp, and various additives. Fiber-cement products offer several advantages over other types of materials, such as wood siding, because they are weatherproof, relatively inexpensive to manufacture, fire-resistant, and invulnerable to rotting or insect damage.
Most commercial fiber-reinforced cement siding products are made using the Hatsheck process. The Hatsheck process was initially developed for the production of asbestos composites, but it is now used for the manufacture of non-asbestos, cellulose fiber reinforced cement composites. In this process, bales of unbleached cellulose pulp fibers are re-pulped in water to provide substantially singulated fibers. The re-pulped fibers are refined and then mixed with cement, silica sand, clay, and other additives to form a mixture. The fiber-cement mixture is deposited on a felt band substrate, vacuum dewatered, layered and in some cases pressed, and then cured to form a fiber reinforced cement matrix in sheet form. The form may have the appearance of standard beveled wood siding.
Other commonly used fiber cement manufacturing processes known to those skilled in the art are: the Magnani process, extrusion, injection molding, hand lay-up, molding and the Mazza pipe process.
Cellulose pulp fibers have two roles in the manufacture of fiber cement products.
Cellulose pulp fibers act as a filter medium in the cement mixture slurry during the drainage process on the forming wire to help retain cement and silica particles while the excess water is being removed from the cement suspension. If there is no filter medium then a great deal of the solids from the slurry will be lost with the water during the drainage process. The purpose of the filter medium is to retain the cement mixture within the product while removing the water.
The fibers also reinforce the cement product. The fiber cement board manufacturers want good strength and good flexibility in the cement board. Strength is indicated by the modulus of rupture of the board. Flexibility is shown by the deflection of the board at maximum load. Maximum load is the amount of force that can be applied to the board before it breaks. Deflection at maximum load is how far the board deflects from the horizontal plane of the board before breaking in three point bending.
A standard against which other cellulose chemical pulp fibers are measured is the Douglas fir unbleached chemical pulp fiber. Other fibers must be comparable with Douglas fir unbleached chemical pulp fiber in modulus of rupture, maximum load and deflection at maximum load if they are to be considered for use in fiber cement board.
Fiber cement boards made with bleached cellulose pulp fibers usually have high strength but are brittle, resulting in poor flexibility. These boards tend to break if flexed and also tend to break when nailed. It would be advantageous to provide a fiber cement board made with bleached cellulose chemical pulp fibers that exhibits both high strength and good flexibility.